The telephone network (commonly referred to as the public switched telephone network or "PSTN") consists of a vast network of interconnected telephone switching facilities. Typically, telephone equipment such as a telephone, a fax machine or a modem connects to the PSTN via a telephone facility referred to as the local exchange or central office ("CO"). Each CO, in turn, connects to one or more switching facilities in the PSTN. Through this arrangement, telephone equipment may place a call that is routed through a CO, through the network, through another CO and, finally, to other telephone equipment.
The portion of the network between the local exchange and customer premises equipment (e.g., telephone equipment installed, for example, at a customer's residence or place of business) is known as the access network (e.g., a local loop). The access network typically consists of copper wires, fiber optic cable, coaxial cable or a combination of these or other components. These components may carry analog signals or digital signals, either of which may be used on different portions of the same access network. When an access network includes both analog and digital portions, the digital portion of the access network may terminate in a connection box located outside of the customer's premises. Copper wire pairs run from the connection box, through the customer premises (e.g., the telephone customers building), to the customer premises equipment ("CPE").
In general, it may be relatively difficult to eavesdrop on a given customer's communications outside of the access network. The PSTN trunks that connect the CO to other switching equipment typically carry multiplexed digital signals. Here, a single trunk (e.g., a copper wire or a fiber optical cable) simultaneously passes the signals for a large number of calls. Thus, it is more difficult for an eavesdropper to determine which trunks in the CO and the PSTN are transmitting a specific customer's communications. Moreover, it may be relatively difficult for an eavesdropper to extract these signals in the event the eavesdropper is successful in identifying the customer's trunk.
In contrast, the access network is more susceptible to eavesdropping. Given the route of the access network through public spaces, eavesdroppers may have relatively easy access to a customer's access network at some point along its route to the CO. Thus, it may be relatively easy to identify an access network of a specific customer and tap the wire. In particular, the portion of the access network that runs over copper wires outside of the customer's premises is especially susceptible to eavesdropping.
Some conventional systems use encryption devices in an attempt to provide secured communications over telephone networks. Typically, an encryption/decryption device is connected to the telephone equipment at each end of the call. For example, signals from a telephone at one end of a call are routed to one of these devices, encrypted, then sent to the PSTN. The encrypted signals are routed through the PSTN and, eventually, to another device. That device decrypts the signals and provides the decrypted signals to the telephone at the other end of the call.
Systems such as the one just described have a several drawbacks. For example, both customers must have the encryption/decryption equipment. This requires the parties to set up the system ahead of time. Thus, these systems are not very cost effective except for parties that make a relatively large number of calls to one another. In addition, in some of these systems, the callers must manually establish the secured connection after calling the other party. This may be cumbersome and may enable eavesdroppers to determine the telephone number of the called party or the calling party with relative ease. Moreover, in many of these systems, calls to destinations that do not have the encryption/decryption equipment are unsecured. In view of these and other drawbacks, a need exists for a more effective method of providing secured communications over telephone networks.